Method to improve uniformity and reduce excess undercuts during chemical etching in the manufacture of solder pads

ABSTRACT

Reduced undercutting of a titanium-tungsten layer in a ball limiting metallurgy (BLM) is achieved in the preparation of solder ball interconnect structures by removing metal oxide film which forms on the titanium-tungsten layer and etching the titanium-tungsten layer in different steps. Removing the metal oxide with an acid solution prior to etching the titanium-tungsten layer provides for a more uniform etch of the titanium-tungsten layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to computer chip manufactureand, more particularly, to improving the manufacture of interconnectstructures in the form of solder bumps used in "flip chip"interconnects, by removing oxide that occurs during the manufacturingprocess.

2. Background Description

"Flip-chip" interconnects comprise an array of solder balls deposited onmetallic layers on one side of an integrated circuit (IC) chip. Thesestructures are sometimes referred to as Controlled, Collapse, ChipConnection (C4) structures, and the solder balls are designed to matewith metallic pads on a chip carrier or substrate having metallizedwiring. In the practice of the present invention, the metallic layers,called Ball Limiting Metallurgy (BLM), on which the solder balls areformed comprise layers of titanium-tungsten and phasedchromium-copper/copper.

Titanium-tungsten is used as an adhesion layer which electricallycontacts interconnects formed in a surface of the chip. The processsteps in electrochemical fabrication of complete "flip-chip"interconnects involve:

1. Sputter deposition of seed layers (titanium-tungsten and phasedchromium-copper/copper);

2. Application of photoresist and its patterning;

3. Electrodeposition of solder;

4. Stripping the photoresist;

5. Etching of the seed layers; and

6. Reflow.

The seed layer etching is a two step process that consists of firstremoving the copper and chromium-copper by electrochemical etching andthen removing the titanium-tungsten by chemical etching. The seed layerswhich remain underneath the solder balls after the two step etchingprocess forms the BLM.

The chemical etching of the titanium-tungsten is preferably performed bya hydrogen peroxide (H₂ O₂) based etchant. The chemical etchant has tofirst remove an oxide film which forms on the titanium-tungsten duringthe electrochemical etching of the copper and chromium-copper beforeetching the actual titanium-tungsten metal layer. The variation in theoxide film across the wafer as well as from wafer to wafer leads todifferences in the time required for removing the oxide film andinitiation of the actual titanium-tungsten metal removal. Since theoxide etch rate differs from the metal etch rate, the result isnon-uniform etching and wide variations in the amounts oftitanium-tungsten undercuts.

The resulting cross-sectional profile of the BLM structure is criticalto building a sound and robust interconnect pad. The undercut or processbias of the titanium-tungsten layer is determined during the wetchemical etch process. The resulting diameters of the copper and phasedchromium-copper layer are easily measured under a microscope on testsites or partial chips. The resulting diameter of the titanium-tungstenportion of the BLM structure is not so easily measured because it ishidden by the overlaying phased chromium-copper layer. Excesstitanium-tungsten undercut must be controlled by process design in orderto ensure a mechanically sound BLM structure.

The titanium-tungsten seed layer of the BLM is typically on the order of1000Å and consists of 90% tungsten and 10% titanium. Titanium-tungstenundercuts due to wet etching have been measured to be in a range from 1micron to 9 microns relative to the phased chromium-copper layer of theBLM. The variation in the titanium-tungsten undercut is seen across thewafers as well as from wafer to wafer. Factors affecting the amount oftitanium-tungsten undercut include etch solution composition,titanium-tungsten layer thickness, and amounts of etch resistant metaloxides formed on the titanium-tungsten surface.

The amount of titanium-tungsten undercut cannot be measurednondestructively. Measuring of titanium-tungsten undercut can alsosignificantly add to the cost. Therefore, a seed layer etching processthat ensures minimal titanium-tungsten undercuts (e.g., less than orequal to 1 micron) is highly desirable.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to prevent excessiveundercutting of titanium-tungsten layer relative to phasedchromium-copper layer in ball limiting metallurgy (BLM), particularly inthe area of the interconnect pad.

According to this invention, an oxide film formed on thetitanium-tungsten layer during electrochemical etching of copper andchromium-copper layers is removed prior to etching thetitanium-tungsten, resulting in a uniform etch rate across the wafersurface and reducing the over-etching underneath the interconnect pad tobetween zero and 1 micron. The metal oxide film on the titanium-tungstenlayer is removed by immersing the wafer in an acid solution immediatelyprior to the chemical etching of the titanium-tungsten seed layer.Because the etch rates of the oxide film and the titanium-tungsten metalare different, removal of the oxide film prior to the metal etch processhas the effect of improving the metal etch uniformity both across thewafer and in the vicinity of the interconnect pad.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a flow chart showing the process for preparing lead-tininterconnects including the step of the present invention;

FIG. 2 is a cross-sectional view of an interconnect pad followingelectroplating of lead-tin pad;

FIG. 3 is a cross-sectional view of an interconnect pad afterelectrochemical etching of seed layers; and

FIG. 4 is a cross-sectional view of an interconnect pad after completionof the process, including the inventive step.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there isshown a flow chart showing the process of preparing C4 structures (i.e.,solder ball on BLM) interconnects, which includes the inventive step.First, as represented by block 1, seed layer deposition occurs. Thisincludes sputter deposition of titanium-tungsten, phasedchromium-copper, and copper layers. In the second step, represented byblock 2, a thick photoresist is applied. In the third step, representedby block 3, resist images are exposed and developed. Then, asrepresented by block 4, the solder pads are electroplated. In the fifthstep, as represented by block 5, the photoresist is stripped.

FIG. 2 shows a cross-sectional view of an interconnect structureprepared after seed layers have been deposited and a solder pad has beenplated. This is how the interconnect structure would look after stepfive of the flow chart shown in FIG. 1. As can be seen, solder pad 11sits over sputtered copper layer 12, sputtered chromium-copper layer 13,and sputtered titanium-tungsten layer 14.

Returning to FIG. 1, in the sixth step as represented by block 6, copperand phased chromium-copper layers are removed by electrochemicaletching. This step results in uneven growth of oxide on the exposedsurface of the titanium-tungsten layer.

FIG. 3 shows how the interconnect structure would look following thesixth processing step as described in connection with block 6 of FIG. 1.

FIG. 3 is a cross-sectional view of the interconnect structure shown inFIG. 2 after electrochemical etching of sputtered copper layer 12 andsputtered chromium-copper layer 13. The electrochemical etch processcreates an overhang 17 of the solder pad 11 over the titanium-tungstenlayer 14 by undercutting the copper and chromium-copper layers 12 and13. As a result of the electrochemical etching, an oxide film 15 formsover at least a portion of the titanium-tungsten layer 14. The oxidefilm is thin or missing on the portion 16 of the titanium-tungsten layerwhich is under the overhang 17 of the solder pad 11. The oxides aremostly tungsten oxides due to the composition of the underlying metallayer.

Returning to FIG. 1, the next step, as represented by block 7, is toremove the oxide that forms on the titanium-tungsten layer with an acidsolution. Immersion in the acid solution removes the metal oxide on thesurface of the exposed titanium-tungsten layer formed duringelectrochemical etching and thus reduces the amount of undercuttingwhich occurs during etching of the titanium-tungsten layer 14. Suitableacids which may be used to remove the oxide include methane sulfonicacid (MSA) and sulfuric acid (H₂ SO₄). Finally, as represented by block8, the titanium-tungsten is removed by a conventional wet etch.

FIG. 4 shows a cross-sectional view of the interconnect structure afterthe final step of the process as it is in block 8 of FIG. 1. Followingwet etching of the titanium-tungsten layer 14, a small undercut 18 ofthe titanium-tungsten appears beneath the sputtered chromium-copperlayer 13. This undercut typically measures between zero and 1 micron.Without the inventive step of removing the oxide from thetitanium-tungsten layer before etching the titanium-tungsten layer, theundercut would measure as much as 10 microns. The wet etching step doesleave some detached titanium-tungsten in the form of ring 20. Thetitanium-tungsten ring does not have any effect on the structure anddoes not need to be removed.

In practice, another source of variation in the oxide that forms on thetitanium-tungsten layer producing wafer to wafer variation has beenobserved due to time delays between the copper/chromium-copperelectrochemical etching step and the titanium-tungsten chemical etchingstep. Excessive delay between the two etching steps with thetitanium-tungsten metal layer exposed to ambient conditions leads tofurther growth of oxide film and can contribute to wafer to wafervariation. This is the same oxide formed during electrochemical etching.Because the oxide is the same, the oxide removal step permits anexcessive delay between two etching steps. The inventive step ofremoving oxide from the titanium-tungsten layer should be performed justprior to titanium-tungsten etch in order to avoid the formation ofanother oxide film on the titanium-tungsten layer.

This invention separates the step of removal of oxide from thetitanium-tungsten layer and the step of etching the titanium-tungstenlayer such that the oxide removal step does not etch anytitanium-tungsten metal. This involves the removal of the oxide on topof the titanium-tungsten layer first immediately followed by etching ofthe titanium-tungsten metal layer. The oxide removal is carried out in asolution that is compatible with the different metals and dielectricspresent on the interconnect wafers and which can effectively remove theoxide films. These materials include copper, chromium-copper,titanium-tungsten, lead-tin (solder), aluminum-0.5% copper, polyimide,silicon oxide and silicon nitride. The preferred solutions are acidicsolutions of H₂ SO₄ and MSA. The preferred concentrations of thesolutions are about 1% to about 30% H₂ SO₄ and about 3N to about 4N MSA.The concentration of the acid solution simply affects the speed withwhich the oxide is removed. Using a higher concentration of acid willresult is faster oxide removal. This process does not attack any of themetals or the dielectric materials and is effective in removing theoxide film on top of the titanium-tungsten layer. The oxide removal isthen immediately followed by etching of the titanium-tungsten layer by,e.g., a H₂ O₂ based etchant.

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is as follows:
 1. A method of improvinguniformity and reducing excess undercuts in the manufacture of solderpad interconnects comprising the steps of:providing a substrate having alayer of ball limiting metallurgy, at least a portion of said layerhaving a metal oxide film thereon; and removing said metal oxide filmwith an acid solution prior to etching selected portions of the layer.2. The method of improving uniformity and reducing excess undercuts inthe manufacture of solder pad interconnects as recited in claim 1wherein said acid solution is selected from the group consisting ofsulfuric acid solutions and methane sulfonic acid solutions.
 3. Themethod of improving uniformity and reducing excess undercuts in themanufacture of solder pad interconnects as recited in claim 1 furthercomprising the step of etching said ball limiting metallurgy, whereinthe etching step is performed immediately after the step of removingsaid metal oxide film.
 4. The method of improving uniformity andreducing excess undercuts in the manufacture of solder pad interconnectsas recited in claim 3 wherein the step of etching of said ball limitingmetallurgy is performed with a hydrogen peroxide based etchant.
 5. Amethod of improving uniformity and reducing excess undercuts in themanufacture of solder pad interconnects comprising the steps of:sputterdepositing a seed layer of a first metal; depositing at least a secondmetal layer on said seed layer; depositing solder onto said second metallayer; electrochemical etching said second metal layer; removing with anacid solution a metal oxide film formed on surfaces of said seed layerexposed during said electrochemical etching of said second metal layer;and etching said seed layer with an etchant different than said acidsolution.
 6. The method of improving uniformity and reducing excessundercuts in the manufacture of solder pad interconnects as recited inclaim 5 wherein said acid solution is selected from the group consistingof sulfuric acid solutions and methane sulfonic acid solutions.
 7. Themethod of improving uniformity and reducing excess undercuts in themanufacture of solder pad interconnects as recited in claim 5 whereinthere is an delay between the steps of electrochemical etching andremoving the metal oxide film.
 8. The method of improving uniformity andreducing excess undercuts in the manufacture of solder pad interconnectsas recited in claim 5 wherein etching of said seed layer is performedwith a hydrogen peroxide based etchant.